Compared to their bulk counterparts, guided-wave electro-optic modulators offer a drastic reduction in the required driving power combined with a bandwidth extending into the 40 GHz band. A variety of electro-optic materials have been studied with a view towards their use for mass-production of modulators. Ferroelectric crystals, such as lithium niobate and the like, are the materials of choice for the vast majority of practical applications on account of their excellent electro-optic optic properties, high optical quality, and availability in large-size wafers from multiple vendors and at a reasonable cost. Of the multitude of guided-wave modulators proposed and developed thus far, traveling-wave modulators are the preferred choice over lumped-electrode modulators, especially at high frequencies.
A conventional traveling wave modulator 100 will now be described, with respect to FIG. 1. The traveling-wave modulator 100 of FIG. 1 consists of an optical waveguide structure formed in an electro-optic chip, and a traveling-wave electrode structure.
Electro-optic chip 101 as shown in FIG. 1, is typically a lithium niobate chip. The typical optical waveguide structure of a modulator is formed in a lithium niobate substrate by thermally diffusing titanium into the substrate or by ion (proton) exchange.
In the optical waveguide structure of a modulator, and as shown in FIG. 1, a Mach-Zehnder interferometer (MZI) 102 is typically disposed. A Mach-Zehnder modulator includes an interferometer 102 having an input waveguide 103 and arms 104, 105 that branch from the input waveguide 103, and an output waveguide 106 at the junction of the arms 104, 105. Typically, an optical signal is directed into and propagates in the input waveguide 103, and is split between the arms 104, 105 so that approximately one-half of the input optical signal propagates in each of the interferometer arms 104, 105. A modulating signal is applied to the modulator in order to change the effective refractive indices of the interferometer arms and to introduce a relative phase shift between the two optical signals. The phase-shifted optical signals combine at the output waveguide 106 and produce intensity modulation. Depending on the relative phase shift between the two optical signals, they may interfere either constructively or destructively. The output of the modulator is thus, an intensity modulated optical signal. A relative phase-shift between the optical signals in the arms 104, 105 of approximately π is required to switch the output of the modulator between adjacent on and off states.
The electrode structure shown in FIG. 1 is a microwave Coplanar Waveguide (CPW) aligned with respect to the waveguide structure in such a way that the interferometer arms 104, 105 are positioned in the electrode gaps 107 and run alongside the gap edges (in X-cut devices). A CPW electrode structure includes a hot central (signal) electrode 108 and two ground planes or electrodes 109, 110 formed on opposite sides of the central electrode 108. Microwave (modulating) signals are provided to the signal electrode 108 from a microwave source 111, via a connector and a microwave cable. The widths of the gap 107 and hot electrode 108 are tapered at the input and output. Advantageously, a CPW structure with properly chosen widths of the gap 107 and hot electrode 108 at the modulator input has a microwave field distribution closely matching that of a coaxial cable connecting the modulator to the microwave source 111.
FIG. 1 shows an intensity modulator 100 with the so-called “push-pull” configuration, wherein the electric field crossing one interferometer arm 104 is opposite to that crossing the other 105. As such, the electro-optic phase shifts induced in the two arms 104, 105 are of equal magnitude but opposite sign. Effectively, the intensity modulator 100 incorporates two phase modulators, represented by the two arms 104, 105, which are driven 180 degrees out of phase with respect to each other. The outputs of the two phase modulators are combined to result in an intensity modulation that depends on the relative phase difference between the arms 104, 105. In the push-pull configuration, the phase difference doubles compared to the phase shift in each arm and the half-wave voltage of the modulator is therefore halved, which represents a significant advantage over other schemes.
The main factors limiting the performance of traveling-wave modulators as that shown in FIG. 1, are RF loss of the CPW electrode and the velocity mismatch between the optical waveguide mode and the microwave CPW mode. There has been a significant amount of research and development effort directed at further improvement of traveling-wave modulators, specifically, at the minimization of the RF power required for a given level of optical modulation depth at a certain microwave frequency. Optimization of performance is typically attempted by varying the parameters that define the modulator cross-section, namely the hot electrode width, inter-electrode gap size, and the thickness of the electrode and buffer layer. This task is complicated and the degree of improvement is limited by the numerous trade-offs that relate the cross-section parameters to various performance characteristics. For example, a lower drive voltage can be achieved at low frequencies by thinning the buffer layer and narrowing the gap, but an increased velocity mismatch and lower line impedance will cause the bandwidth and microwave return loss to decrease.
In many applications, a phase, rather than intensity, modulator is required. As seen in FIG. 2, in this case a modulator with a CPW structure 200 cannot provide a push-pull configuration, and exactly one half of the total RF power launched into the CPW electrode is lost in the gap 201 not used by the waveguide 202. As a result, the phase modulator 200 has twice the half-wave voltage of the corresponding push-pull MZI 100.
The coplanar strip (CPS) structure shown in FIG. 3 may be a better choice for a phase modulator 300 because its higher impedance is close to the coaxial feed line from an RF source 307 (typically 50 Ohm). The coplanar strip (CPS) electrode structure 300 includes a hot electrode 302 to which a modulating signal is applied, and a single ground plane or grounded electrode 303 on one side of the hot electrode 302. Compared to the CPW electrode described above, the CPS structure has higher impedance for a given ratio of the hot electrode 302 width to the gap 306 width. As such, better impedance matching between the 50 Ohm feed line from the RF source 307 and the CPS electrode can be accomplished, resulting in lower microwave reflection and more microwave power reaching the electro-optic interaction length 308 and thus producing more optical modulation for a given level of the input microwave power level. Furthermore, the CPS structure by itself is known to typically have a better electro-optic overlap integral, and thus a lower drive voltage, than the CPW electrode.
However, the electric field distribution of a CPS line is rather mismatched from that of the feed line. This circumstance limits RF coupling efficiency, causes some fraction of RF power to be launched into the modulator chip 305 in the form of substrate modes, and eventually increases driving voltage and RF reflection.
On balance, it can be seen than the CPW structure is better suited for coupling in and out of a coaxial cable, while the CPS structure is better suited for electro-optic modulation. Accordingly, a technique of improving the optical modulation efficiency and minimizing microwave reflection of guided-wave modulators that could combine the benefits of the CPW or CPS electrode structure types, while avoiding the drawbacks associated with each type, is highly desired.